Fuel cells are intensively researched and developed in various types ranging from medium temperature to high temperature operating type, such as phosphoric acid fuel cell (PAFC), molten carbonate fuel cell (MCFC), and solid oxide fuel cell (SOFC), as power generation system applicable in wide uses from small-scale electric power generation to large plants, and practical use is promoted in some of them.
On the other hand, fuel cells designed for use in small-scale independent power supply or portable power supply that can operate at a relatively low temperature near an ordinary temperature are also researched and developed. As a typical example thereof, the alkaline fuel cell (AFC) is known well, and recently, the PEMFC using the polymer electrolyte (PE) such as NAFION.RTM. membrane or other cation exchange membrane as hydrogen ion (proton) conductive material is drawing attention. In the PE of the PEMFC initially mounted on the manned spacecraft Gemini, styrene-divinyl benzene sulfonic acid group polymer membrane was used. However, since this membrane was likely to elevate in the internal resistance due to local dehydration phenomenon and low in heat resistance, it was replaced by NAFION.RTM. membrane. By repeated improvement of characteristic and reduction of cost, the PEMFC is not only used as the power supply aboard the spacecraft, but also noticed recently for consumer use such as driving power source of electric vehicle and boat and portable power supply.
In the PEMFC, pure hydrogen or reforming hydrogen from methanol or the like is used for the fuel as the reducing agent of the anode, and pure oxygen or oxygen in the air is used as the oxidizing agent of the cathode.
The electrode reaction at the fuel electrode (anode) side is EQU H.sub.2.fwdarw.2H.sup.+ +2e.sup.- (1-1)
and therefore hydrogen is consumed. The electrode reaction occurring at the oxygen electrode (cathode) side is EQU 1/20.sub.2 +2H.sup.+ +2e.sup.-.fwdarw.H.sub.2 O (1-2)
and oxygen is consumed and water vapor is generated. The overall reaction is expressed as EQU H.sub.2 +1/20.sub.2.fwdarw.H.sub.2 O (1-3)
and an electric power is generated.
Reactions of gas electrodes expressed in formulas (1-1) and (1-2) take place, as known well, in the three-phase zone near the three-phase interface in which reductive or oxidative gases, PE which is an ion conductive solid, and powdered carbon of electronic conductive solid on which the catalyst is carried contact with each other. As the electrode material in PEMFC, in both anode and cathode, powdered carbon is used such as graphite and acetylene black carrying Pt catalyst mixed with PE. As the catalyst metal of anode, iridium (Ir) may be used instead of Pt.
The gas electrode reaction in the anode and cathode occurs on the noble metal catalyst composed of Pt and/or Ir carried on powdered carbon, but all catalyst cannot contribute to the reaction. Only the catalyst porously covered with PE and capable of contacting with reaction gas can contribute to the reaction. In other words, if contacting with reaction gas, the catalyst not covered with PE cannot contribute to the reaction. That is, when the specific surface area available for reaction is large in the noble metal catalyst porously covered with PE and allowing reaction gas to diffuse easily, polarization becomes smaller and the discharge characteristics of the electrode are enhanced.
The specific surface area available for reaction of noble metal catalyst is expressed in formula (2-1).
Specific surface area available for reaction of noble metal catalyst EQU (m.sup.2.multidot.g.sup.-1)=total specific area of noble metal catalyst (m.sup.2.multidot.g.sup.-1)-specific surface area of noble metal catalyst not covered with PE(m.sup.2.multidot.g.sup.-1) (2-1)
Together with this specific surface area available for reaction of noble metal catalyst, the utilization of noble metal catalyst which is percentage of specific surface area available for reaction of total specific area of noble metal catalyst is an extremely important factor for designing of catalyst layer of PEMFC electrode. ##EQU1##
Hitherto, the total specific area of noble metal catalyst such as Pt was measured by first reducing the catalyst composed of powdered carbon on which the noble metal is carried by H.sub.2 at 400.degree. C., and then determining from the adsorption amount of carbon monoxide (CO). This method of measurement makes use of the nature that CO is adsorbed only on the Pt catalyst reduced by H.sub.2, but there was a problem because the H.sub.2 reducing temperature was set at 400.degree. C. When using the noble metal carried catalyst as the electrode, PE was mixed, but as the PE which is an organic compound is decomposed in the reducing process, the value of measurement becomes inaccurate. To the contrary, in order to avoid decomposition of PE, if the H.sub.2 reducing temperature is set too low, the total specific area of the catalyst becomes larger perhaps because CO adsorbs on the surface of other substance than the noble metal catalyst. It was hence necessary to select an appropriate H.sub.2 reducing temperature.
Moreover, conventionally, the specific surface area available for reaction of noble metal catalyst was measured from the adsorption and desorption waves of hydrogen atom by cyclic voltammetric method in an electrolyte solution composed of, for example, diluted sulfuric acid (H.sub.2 SO.sub.4), but using the electrode prepared by using powdered carbon carrying noble metal, according to formula 2-3. ##EQU2##
where
S.sub.cv [m.sup.2.multidot.g.sup.-1 Pt]: specific surface area of Pt catalyst PA1 Q.sub.H(a) [C]: adsorption and desorption coulombic amount of hydrogen atom PA1 W.sub.Pt [g]: total Pt amount in electrode PA1 C [C.multidot.m.sup.-2 ]: adsorption and desorption coulombic amount of hydrogen atom per PA1 1 m.sup.2 of surface area of Pt catalyst
In the electrode for PEMFC, it is hard to cover the entire surface of the catalyst with PE which is, by nature, large in molecular diameter. The molecular diameter of the electrolyte in the electrolyte aqueous solution used in the conventional cyclic voltammetric method is smaller than that of PE and the permeability is high, and it permeates further and contacts even with the catalyst not contributing to reaction by nature because it is not covered with PE, and as a result the specific surface area available for reaction of the noble metal catalyst appears to be larger.
It is hence an object of the invention to present a novel measuring method capable of completely solving the problems of the conventional measuring methods described above. That is, without employing the conventional cyclic voltammetric method, by the CO adsorption method aftter H.sub.2 reduction at specified temperature, the total specific area of the noble metal catalyst and the specific surface area of the noble metal catalyst not covered with PE and hence not contributing to the reaction are measured, the latter measurement is subtracted from the former measurement to determine the specific surface area available for reaction of the noble metal catalyst, and the catalyst layer of the PEMFC electrode defined according to the practical situation is designed depending on the utilization of the noble metal catalyst obtained therefrom, and the practical value is extremely large.